Photoacoustic image generation apparatus and acoustic wave unit

ABSTRACT

A pulse laser beam is emitted in a desired wavelength sequence using a laser source unit. A Q switch and a birefringent filter are inserted into an optical resonator including a pair of mirrors and facing each other with a laser rod interposed therebetween. The birefringent filter changes an oscillation wavelength of the optical resonator in association with rotational displacement. A trigger control circuit rotates the birefringent filter at a predetermined rotation speed depending on the number of wavelengths included in the wavelength sequence of the pulse laser beam to be emitted. In addition, the trigger control circuit irradiates the laser rod with excitation light, and turns on the Q switch at a timing when a rotational displacement position of the birefringent filter is set to a position corresponding to the wavelength of the pulse laser beam to be emitted, to cause the pulse laser beam to be emitted.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photoacoustic image generationapparatus and an acoustic wave unit, and more particularly, to aphotoacoustic image generation apparatus, which when a test object isirradiated with a laser beam having a plurality of wavelengths,generates a photoacoustic image on the basis of photoacoustic signalsdetected with respect to the respective wavelengths, and an acousticwave unit.

2. Description of the Related Art

Hitherto, for example, as disclosed in JP2005-21380A and A High-SpeedPhotoacoustic Tomography System based on a Commercial Ultrasound and aCustom Transducer Array, Xueding Wang, Jonathan Cannata, DerekDeBusschere, Changhong Hu, J. Brian Fowlkes, and Paul Carson, Proc. SPIEVol. 7564, 756424 (Feb. 23, 2010), a photoacoustic image formingapparatus that forms an image of the inside of a living body using aphotoacoustic effect has been known. In the photoacoustic image formingapparatus, a living body is irradiated with pulsed light such as a pulselaser beam. Body tissues absorbing energy of the pulsed light expand involume inside the living body irradiated with the pulsed light, and thusacoustic waves are generated. It is possible to detect the acousticwaves using an ultrasonic probe or the like, and to form a visible imageof the inside of the living body on the basis of the detected signal(photoacoustic signal). In a photoacoustic image forming method,acoustic waves are generated in a specific light absorber, and thus itis possible to form an image of specific tissues in the living body, forexample, blood vessels.

Incidentally, many of body tissues have an optical absorption propertyvarying depending on a wavelength of light, and generally, the opticalabsorption property is unique for each tissue. For example, FIG. 12illustrates molecular absorption coefficients of oxygenated hemoglobin(hemoglobin combined with oxygen: oxy-Hb) which is contained in a largeamount in an artery of a human and deoxygenated hemoglobin (hemoglobinnot combined with oxygen: deoxy-Hb) which is contained in a large amountin a vein, depending on light wavelengths. An optical absorptionproperty of an artery corresponds to that of oxygenated hemoglobin, andan optical absorption property of a vein corresponds to that ofdeoxygenated hemoglobin. There is known a photoacoustic image formingmethod of irradiating blood vessel parts with a light beam having twodifferent types of wavelengths and of distinctively forming images of anartery and a vein (for example, see JP2010-046215A), using a differencein light absorptivity according to the wavelengths.

Herein, with regard to a variable wavelength laser, JP2009-231483Adiscloses that a laser beam having a desired wavelength is obtained bydisposing an etalon or a birefringent filter as a wavelength selectionelement within an optical resonator and adjusting the rotation anglethereof. In addition, JP2000-105464A discloses that an etalon aswavelength selection means is disposed within an optical resonator andthat the etalon is scanned at a constant speed. JP2000-105464A disclosesthat laser oscillation is performed only when a transmission wavelengthof the etalon is consistent with longitudinal mode oscillation of alaser beam and that the oscillation of the laser beam is performed in apulsed manner when a scanning speed of the etalon is increased.

SUMMARY OF THE INVENTION

In JP2009-231483A, in order to switch and emit a laser beam having aplurality of wavelengths, it is necessary to adjust a rotation angle ofthe etalon or the birefringent filter at every laser emission. Inphotoacoustic imaging, for example, it is considered that when a testobject is irradiated with pulse laser beams having a first wavelengthand a second wavelength, the wavelength selection element is adjusted toirradiate the test object with the laser beam having the firstwavelength, the detection of all photoacoustic signals of the laser beamhaving the first wavelength is terminated, and then the wavelengthselection element is adjusted so as to emit the laser beam having thesecond wavelength, and the test object is irradiated with the laser beamhaving the second wavelength. In the photoacoustic imaging, an objecthaving a movement such as a human is often selected. Therefore, when anobject moves during switching from the first wavelength to the secondwavelength, mismatching may occur between a photoacoustic signal at thetime of irradiation with the laser beam having the first wavelength anda photoacoustic signal at the time of irradiation with the laser beamhaving the second wavelength.

In the photoacoustic imaging, in terms of the prevention of theabove-mentioned mismatching, for example, it is considered that theirradiation with a laser beam may be performed by switching the firstwavelength and the second wavelength for each pulse. In other words, forexample, it is considered that the irradiation with the laser beam maybe repeatedly performed in a predetermined wavelength sequence includingthe first wavelength and the second wavelength in this order.JP2000-105464A discloses a laser device that changes a wavelength of alaser beam with which the irradiation is performed for each pulse.However, in JP2000-105464A, since a laser is oscillated only when thetransmission wavelength of the etalon is consistent with thelongitudinal mode oscillation of the laser beam, a laser beam havingonly a specific wavelength sequence can be obtained, and a laser beamhaving any wavelength sequence cannot be obtained.

The invention is contrived in view of such situations, and an objectthereof is to provide an acoustic wave unit capable of emitting a pulselaser beam in a desired wavelength sequence from a wavelength variablelaser light source, and a photoacoustic image generation apparatusincluding the acoustic wave unit.

In order to achieve the above-described object, the invention provides aphotoacoustic image generation apparatus including: a laser source unitthat sequentially emits a plurality of pulse laser beams in apredetermined wavelength sequence having at least two differentwavelengths, the laser source unit including a laser rod, an excitationlight source that irradiates the laser rod with excitation light, anoptical resonator that has a pair of mirrors facing each other with thelaser rod interposed therebetween, a Q switch which is inserted into theoptical resonator, and a birefringent filter which is inserted into theoptical resonator and changes an oscillation wavelength of the opticalresonator in association with rotational displacement of thebirefringent filter; and an acoustic wave unit that generates aphotoacoustic image, the acoustic wave unit including detection unitthat detects a photoacoustic signal generated within an object when theobject is irradiated with the pulse laser beam having each wavelengthincluded in the predetermined wavelength sequence and generates piecesof photoacoustic data corresponding to the respective wavelengths,intensity ratio extraction unit that extracts a magnitude relationbetween relative signal intensities of the pieces of photoacoustic datacorresponding to the respective wavelengths, photoacoustic imageconstruction unit that generates the photoacoustic image on the basis ofthe extracted magnitude relation, and a trigger control circuit thatcauses the laser rod to be irradiated with excitation light from theexcitation light source while rotating the birefringent filter at apredetermined rotation speed depending on the number of wavelengthsincluded in the wavelength sequence, and after the irradiation with theexcitation light, turns on the Q switch at a timing when a rotationaldisplacement position of the birefringent filter is set to a positioncorresponding to the wavelength of the pulse laser beam to be emitted tocause the pulse laser beam to be emitted.

In the invention, the predetermined rotation speed may be determined onthe basis of a change characteristic of the oscillation wavelength withrespect to the rotational displacement position in the birefringentfilter, the number of wavelengths included in the wavelength sequence,and the number of times of emission of the pulse laser beam per unittime.

When the number of times of a free spectral range repeated during onerotation is set to k[times/rotation], the number of wavelengths includedin the wavelength sequence is set to n[pieces], and the number of timesof emission of the pulse laser beam per unit time is set tom[times/second], the predetermined rotation speed of the birefringentfilter may be determined as a value calculated by a relation ofv=m/(k×n)[rotations/second].

In the invention, the trigger control circuit may continuously rotatethe birefringent filter in a predetermined direction at thepredetermined rotation speed.

The trigger control circuit may determine a timing at which theexcitation light is irradiated and a timing at which the Q switch isturned on, on the basis of birefringent filter state informationindicating the rotational displacement position of the birefringentfilter.

When the birefringent filter state information is set to informationindicating a position obtained by subtracting the amount of rotationaldisplacement of the birefringent filter during a period of time requiredfor the excitation of the laser rod from a position of the birefringentfilter which corresponds to the wavelength of the pulse laser beam to beemitted, the trigger control circuit may cause the laser rod to beirradiated with excitation light.

The trigger control circuit may rotate the birefringent filter so thatthe amount of change in the birefringent filter state information duringa predetermined period of time is set to the amount of change dependingon the predetermined rotation speed.

Moreover, the laser source unit may further include driving unit thatrotates the birefringent filter, rotational displacement detection unitthat detects the rotational displacement of the birefringent filter, anda rotation control unit that controls the driving unit so that theamount of rotational displacement of the birefringent filter which isdetected by the rotational displacement detection unit during apredetermined period of time is set to an amount depending on thepredetermined rotation speed.

The rotational displacement detection unit may output a birefringentfilter state signal indicating the rotational displacement position ofthe birefringent filter to the acoustic wave unit.

The trigger control circuit may output a birefringent filter controlsignal for controlling rotation of the birefringent filter to thedriving unit.

The trigger control circuit may output a flash lamp trigger signal forcontrolling emission of the excitation light to the excitation lightsource on the basis of the birefringent filter state signal, and maycause the laser rod to be irradiated with excitation light from theexcitation light source.

The trigger control circuit may output a Q switch trigger signal forturning on the Q switch when the birefringent filter state signal is setto signal indicating a position of the birefringent filter whichtransmits a wavelength of a pulse laser beam to be emitted, and maycause the pulse laser beam to be emitted from the laser source unit.

The acoustic wave unit may include an AD converting unit that performssampling of the photoacoustic signal with a predetermined samplingperiod, and the trigger control circuit may output a sampling triggersignal indicating a sampling timing of the AD converting unit to the ADconverting unit, in synchronization with an output timing of the Qswitch trigger signal.

The acoustic wave unit may further include intensity informationextraction unit that generates intensity information indicating signalintensity on the basis of the pieces of photoacoustic data correspondingto the respective wavelengths. The photoacoustic image construction unitmay determine a gradation value of each pixel of the photoacoustic imageon the basis of the intensity information and may determine a displaycolor of each pixel on the basis of the extracted magnitude relation.

The predetermined wavelength sequence may include a first wavelength anda second wavelength. The acoustic wave unit may further include complexnumber creation unit that generates complex number data in which one offirst photoacoustic data corresponding to a photoacoustic signal,detected when irradiation with the pulse laser beam having the firstwavelength is performed, and second photoacoustic data corresponding toa photoacoustic signal, detected when irradiation with the pulse laserbeam having the second wavelength is performed, is set to a real partand the other one is set to an imaginary part, and photoacoustic imagereconstruction unit that generates a reconstructed image from thecomplex number data using a Fourier transform method. The intensityratio extraction unit may extract phase information as the magnituderelation from the reconstructed image, and the intensity informationextraction unit may extract the intensity information from thereconstructed image.

The detection unit may further detect reflected acoustic waves withrespect to acoustic waves transmitted to the object to generatereflected acoustic wave data, and the acoustic wave unit may furtherinclude acoustic wave image generation unit that generates an acousticwave image on the basis of the reflected acoustic wave data.

The invention also provides an acoustic wave unit including: detectionunit that detects a photoacoustic signal generated within an object whenthe object is irradiated with a pulse laser beam having each wavelengthincluded in a predetermined wavelength sequence including at least twodifferent wavelengths, and generates pieces of photoacoustic datacorresponding to the respective wavelengths; intensity ratio extractionunit that extracts a magnitude relation between relative signalintensities of the pieces of photoacoustic data corresponding to therespective wavelengths; photoacoustic image construction unit thatgenerates a photoacoustic image on the basis of the extracted magnituderelation; and a trigger control circuit that causes the laser rod to beirradiated with excitation light from an excitation light source whilerotating a birefringent filter which is inserted into an opticalresonator that includes a pair of mirrors facing each other with a laserrod interposed therebetween and changes an oscillation wavelength of theoptical resonator in association with rotational displacement of thebirefringent filter, at a predetermined rotation speed depending on thenumber of wavelengths included in the wavelength sequence, and after theirradiation with the excitation light, turns on a Q switch inserted intothe optical resonator at a timing when a rotational displacementposition of the birefringent filter is set to a position correspondingto the wavelength of the pulse laser beam to be emitted, to cause thepulse laser beam to be emitted.

In a photoacoustic image generation apparatus and an acoustic wave unitof the invention, a birefringent filter which is inserted into anoptical resonator and changes an oscillation wavelength in associationwith rotational displacement of the birefringent filter is rotated at arotation speed depending on the number of wavelength sequences of apulse laser beam to be emitted from a laser source unit, and a Q switchwhich is inserted into the optical resonator is turned on at a timingwhen a rotational displacement position of the birefringent filter isset to a position corresponding to the wavelength of the pulse laserbeam to be emitted. As the rotation speed of the birefringent filterincreases, the speed of the wavelength switching can be increased. Onthe contrary, as the rotation speed thereof decreases, the number ofselectable oscillation wavelengths can be increased. The rotation speedof the birefringent filter of the present invention is controlleddepending on the number of wavelengths included in a wavelengthsequence. Based on such a configuration, the wavelengths of the pulselaser beam to be emitted from the laser source unit can be controlled tohave any wavelength sequence by the acoustic wave unit. In addition, theacoustic wave unit of the present invention determines an emissiontiming of the pulse laser beam, and thus it is not necessary to acquirea signal such as a synchronization signal indicating laser emission fromthe laser source unit, in the start of sampling of a photoacousticsignal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a photoacoustic image generation apparatusaccording to a first embodiment of the invention.

FIG. 2 is a block diagram illustrating a configuration of a laser sourceunit according to the first embodiment.

FIG. 3 is a perspective view illustrating a configuration example of abirefringent filter, driving unit, and rotational displacement detectionunit.

FIG. 4 is a graph illustrating an example of a wavelength transmissioncharacteristic for rotational displacement of the birefringent filter.

FIG. 5 is a graph illustrating an oscillation wavelength characteristicwhen the birefringent filter is rotated at a speed of one rotation persecond.

FIG. 6 is a timing chart illustrating various types of triggers and anemission timing.

FIG. 7 is a flow chart illustrating an operation procedure of thephotoacoustic image generation apparatus according to the firstembodiment.

FIG. 8 is a timing chart illustrating various types of triggers and anemission timing in a case where a wavelength sequence includes sixwavelengths.

FIG. 9 is a block diagram illustrating a photoacoustic image generationapparatus according to a second embodiment of the invention.

FIG. 10 is a block diagram illustrating an operation procedure of thephotoacoustic image generation apparatus according to the secondembodiment.

FIG. 11 is a block diagram illustrating a configuration of a lasersource unit according to a modified example.

FIG. 12 is a graph illustrating molecular absorption coefficients ofoxygenated hemoglobin and deoxygenated hemoglobin depending on lightwavelengths.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the invention will be described in detailwith reference to the accompanying drawings. Meanwhile, in examples ofthe invention, ultrasonic waves are used as acoustic waves, but theacoustic waves may be acoustic waves having an audible frequency byselecting an appropriate frequency according to an object to be testedor measurement conditions. FIG. 1 illustrates a photoacoustic imagegeneration apparatus according to a first embodiment of the invention. Aphotoacoustic image generation apparatus 10 includes an ultrasonic waveprobe (probe) 11, an ultrasonic wave unit 12, and a laser source unit13. The laser source unit 13 emits a pulse laser beam with which a testobject is to be irradiated. The laser source unit 13 emits a pluralityof pulse laser beams in a predetermined wavelength sequence including atleast two different wavelengths. Hereinafter, a description will bemainly given on the assumption that the wavelength sequence includes afirst wavelength and a second wavelength in this order and the lasersource unit 13 emits a pulse laser beam having the first wavelength anda pulse laser beam having the second wavelength in this order.

For example, a wavelength of approximately 750 nm is considered as thefirst wavelength (center wavelength), and a wavelength of approximately800 nm is considered as the second wavelength. Referring to FIG. 12described above, a molecular absorption coefficient of oxygenatedhemoglobin (hemoglobin combined with oxygen: oxy-Hb) which is containedin a large amount in an artery of a human at a wavelength of 750 nm islower than a molecular absorption coefficient of that at a wavelength of800 nm. On the other hand, a molecular absorption coefficient ofdeoxygenated hemoglobin (hemoglobin not combined with oxygen: deoxy-Hb)which is contained in a large amount in a vein at a wavelength of 750 nmis higher than a molecular absorption coefficient of that at awavelength of 800 nm. It is possible to discriminate between aphotoacoustic signal from the artery and a photoacoustic signal from thevein by examining whether a photoacoustic signal obtained at thewavelength of 750 nm is relatively larger or smaller than aphotoacoustic signal obtained at the wavelength of 800 nm, using such aproperty.

The pulse laser beam emitted from the laser source unit 13 is guided toa probe 11 using light guiding means such as an optical fiber, and isirradiated toward a test object from the probe 11. An irradiationposition of the pulse laser beam is not particularly limited, and thepulse laser beam may be irradiated from any place other than the probe11. Ultrasonic waves (acoustic waves) are generated within the testobject by a light absorber absorbing energy of the irradiated pulselaser beam. The probe 11 includes an ultrasonic wave detector. The probe11 includes, for example, a plurality of ultrasonic wave detectorelements (ultrasonic wave vibrators) which are arrangedone-dimensionally, and the acoustic waves (photoacoustic signal) fromthe inside of the test object are detected by the ultrasonic wavevibrators that are arranged one-dimensionally.

The ultrasonic wave unit 12 includes a reception circuit 21, ADconversion unit 22, a reception memory 23, complex number creation unit24, photoacoustic image reconstruction unit 25, phase informationextraction unit 26, intensity information extraction unit 27, detectionand logarithmic transformation unit 28, photoacoustic image constructionunit 29, a trigger control circuit 30, and control unit 31. Thereception circuit 21 receives a photoacoustic signal detected by theprobe 11. The AD conversion unit 22, which is detection unit, samplesthe photoacoustic signal received by the reception circuit 21 andgenerates photoacoustic data which is digital data. The AD conversionunit 22 samples the photoacoustic signal with a predetermined samplingperiod in synchronization with an AD clock signal.

The AD conversion unit 22 stores photoacoustic data in the receptionmemory 23. The AD conversion unit 22 stores, in the reception memory 23,photoacoustic data corresponding to the respective wavelengths of thepulse laser beam emitted from the laser source unit 13. In other words,the AD conversion unit 22 stores, in the reception memory 23, firstphotoacoustic data obtained by sampling a photoacoustic signal detectedby the probe 11 when a test object is irradiated with a pulse laser beamhaving a first wavelength and second photoacoustic data obtained bysampling a photoacoustic signal detected by the probe 11 when the testobject is irradiated with a second pulse laser beam.

The complex number creation unit 24 reads out the first photoacousticdata and the second photoacoustic data from the reception memory 23, andgenerates complex number data in which any one of the firstphotoacoustic data and the second photoacoustic data is set to a realpart and the other one is set to an imaginary part. Hereinafter, adescription will be given on the assumption that the complex numbercreation unit 24 generates the complex number data in which the firstphotoacoustic data is set to a real part and the second photoacousticdata is set to as an imaginary part.

The photoacoustic image reconstruction unit 25 inputs the complex numberdata from the complex number creation unit 24. The photoacoustic imagereconstruction unit 25 performs image reconstruction from the inputcomplex number data using a Fourier transform method (FTA method). Awell-known method of the related art which is disclosed in, for example,a document “Photoacoustic Image Reconstruction-A Quantitative Analysis”Jonathan I. Sperl et al. SPIE-OSA, Vol. 6631 663103 can be applied tothe image reconstruction using the Fourier transform method. Thephotoacoustic image reconstruction unit 25 inputs Fourier transformeddata indicating the reconstructed image to the phase informationextraction unit 26 and the intensity information extraction unit 27.

The phase information extraction unit 26 extracts a magnitude relationbetween relative signal intensities of pieces of photoacoustic datacorresponding to the respective wavelengths. In this embodiment, thephase information extraction unit 26 sets the reconstructed imagereconstructed by the photoacoustic image reconstruction unit 25 to inputdata. In addition, the phase information extraction unit generates, whenthe real part and the imaginary part are compared with each other, phaseinformation indicating how relatively large either of the two parts is,using the input data which is the complex number data. For example, whenthe complex number data is expressed by X+iY, the phase informationextraction unit 26 generates the relation of θ=tan⁻¹(Y/X) as phaseinformation. Meanwhile, when the relation of X=0 is satisfied, therelation of θ=90° is established. When first photoacoustic data (X)constituting the real part is equal to second photoacoustic data (Y)constituting the imaginary part, the phase information satisfies therelation of θ=45°. As the first photoacoustic data becomes relativelylarger, the phase information becomes closer to the relation of θ=0°. Asthe second photoacoustic data becomes larger, the phase informationbecomes closer to the relation of θ=90°.

The intensity information extraction unit 27 generates intensityinformation indicating signal intensity on the basis of the pieces ofphotoacoustic data corresponding to the respective wavelengths. In thisembodiment, the intensity information extraction unit 27 sets thereconstructed image reconstructed by the photoacoustic imagereconstruction unit 25 to input data, and generates the intensityinformation from the input data which is complex number data. Forexample, when the complex number data is expressed by X+iY, theintensity information extraction unit 27 extracts (X²+Y²)^(1/2) asintensity information. The detection and logarithmic transformation unit28 generates an envelope of data indicating the intensity informationextracted by the intensity information extraction unit 27, and thenwidens a dynamic range by performing logarithmic transformation on theenvelope.

The photoacoustic image construction unit 29 inputs the phaseinformation from the phase information extraction unit 26, and inputsthe intensity information after the detection and logarithmictransformation process from the detection and logarithmic transformationunit 28. The photoacoustic image construction unit 29 generates aphotoacoustic image which is a distribution image of a light absorber,on the basis of the input phase information and intensity information.For example, the photoacoustic image construction unit 29 determinesluminance (gradation value) of each pixel in the distribution image ofthe light absorber, on the basis of the input intensity information. Inaddition, for example, the photoacoustic image construction unit 29determines color of each pixel (display color) in the distribution imageof the light absorber, on the basis of the phase information. Thephotoacoustic image construction unit 29 determines color of each pixelon the basis of the input phase information, for example, using therange of phases 0° to 90° in a color map associated with a predeterminedcolor.

Here, since the range of phases 0° to 45° is a range in which the firstphotoacoustic data is larger than the second photoacoustic data, ageneration source of a photoacoustic signal is considered to be a veinthrough which blood flows, the blood mainly containing deoxygenatedhemoglobin in which the amount of absorption of a wavelength of 756 nmis greater than that of a wavelength of 798 nm. On the other hand, sincethe range of phases 45° to 90° is a range in which the firstphotoacoustic data is smaller than the second photoacoustic data, thegeneration source of the photoacoustic signal is considered to be anartery through which blood flows, the blood mainly containing oxygenatedhemoglobin in which the amount of absorption of a wavelength of 756 nmis less than that of a wavelength of 798 nm.

Consequently, as the color map, a color map is used in which colorgradually changes so as to become blue at the phase of 0° and to becomecolorless (white) as the phase approaches 45° and in which colorgradually changes so as to become red at the phase of 90° and to becomewhite as the phase approaches 45°. In this case, in the photoacousticimage, a portion corresponding to the artery can be expressed by red,and a portion corresponding to the vein can be expressed by blue. Onlycolor coding between the portion corresponding to the artery and theportion corresponding to the vein may be performed on the basis of thephase information by maintaining a constant gradation value, withoutusing the intensity information. The image display unit 14 displays thephotoacoustic image generated by the photoacoustic image constructionunit 29 on a display screen.

Subsequently, a configuration of the laser source unit 13 will bedescribed in detail. FIG. 2 illustrates a configuration of the lasersource unit 13. The laser source unit 13 includes a laser rod 51, aflash lamp 52, mirrors 53 and 54, a Q switch 55, a birefringent filter(BRF) 56, driving unit 57, and rotational displacement detection unit58. The laser rod 51 is a laser medium. Examples of the laser rod 51include alexandrite crystal, Cr:LiSAF (Cr:LiSrAlF6), Cr:LiCAF(Cr:LiCaAlF6) crystal, and Ti: Sapphire crystal. The flash lamp 52 is anexcitation light source, and the laser rod 51 is irradiated withexcitation light. Any of light sources other than the flash lamp 52 maybe used as the excitation light source. For example, when the laser rod51 is formed of titanium sapphire, Nd-YAG (SHG) is used as theexcitation light source.

The mirrors 53 and 54 face each other with the laser rod 51 interposedtherebetween, and an optical resonator is constituted by the mirrors 53and 54. The mirror 54 is assumed to be the output side. The Q switch 55and the birefringent filter 56 are inserted into the optical resonator.An insertion loss within the optical resonator rapidly changes from ahigh loss (low Q) to a low loss (high Q) by the Q switch 55, and thus apulse laser beam can be obtained. The birefringent filter 56 changes atransmission wavelength in association with rotational displacement andchanges an oscillation wavelength of the optical resonator. The drivingunit 57 rotates the birefringent filter 56. The rotational displacementdetection unit 58 detects the rotational displacement of thebirefringent filter 56. The rotational displacement detection unit 58outputs BRF state information indicating the rotational displacementposition of the birefringent filter 56 to the ultrasonic wave unit 12.

Referring back to FIG. 1, the control unit 31 controls each unit withinthe ultrasonic wave unit 12. The trigger control circuit 30 rotates thebirefringent filter 56 within the laser source unit 13 at apredetermined rotation speed depending on the number of wavelengthsincluded in a wavelength sequence of a pulse laser beam to be emittedfrom the laser source unit 13. The rotation speed of the birefringentfilter can be determined, for example, on the basis of a changecharacteristic of an oscillation wavelength with respect to therotational displacement position in the birefringent filter 56, thenumber of wavelengths included in the wavelength sequence, and thenumber of times of emission of the pulse laser beam per unit time(interval of time between pulse laser beams).

The trigger control circuit 30 outputs a BRF control signal forcontrolling the rotation of the birefringent filter 56. The driving unit57 of the laser source unit 13 rotates the birefringent filter 56 inresponse to the BRF control signal. For example, the trigger controlcircuit 30 rotates the birefringent filter so that the amount of changein BRF state information during a predetermined period of time is set tothe amount of change depending on a predetermined rotation speed, on thebasis of the BRF control signal.

In addition to the above description, the trigger control circuit 30outputs a flash lamp trigger signal for controlling the emission of theflash lamp 52 to the laser source unit 13, and causes the laser rod 51to be irradiated with excitation light from the flash lamp 52. Thetrigger control circuit 30 outputs the flash lamp trigger signal on thebasis of a BRF state signal. For example, when the BRF state informationis set to information indicating the position obtained by subtractingthe amount of rotational displacement of the birefringent filter 56during a period of time required for the excitation of the laser rod 51from the position of the birefringent filter 56 which corresponds to thewavelength of the pulse laser beam to be emitted, the trigger controlcircuit 30 outputs the flash lamp trigger signal and causes the laserrod 51 to be irradiated with excitation light.

After the irradiation with the excitation light, the trigger controlcircuit 30 outputs a Q switch trigger signal to the Q switch 55 at atiming when the rotational displacement position of the birefringentfilter 56 is set to the position corresponding to the wavelength of thepulse laser beam to be emitted. In other words, the trigger controlcircuit 30 outputs the Q switch trigger signal when the BRF stateinformation is set to information indicating the position of thebirefringent filter 56 which transmits the wavelength of the pulse laserbeam to be emitted. The Q switch 55 rapidly changes the insertion losswithin the optical resonator from a high loss to a low loss (Q switch isturned on) in response to the Q switch trigger signal, and thus thepulse laser beam is emitted from the mirror 54 on the output side.

The trigger control circuit 30 outputs a sampling trigger signal (ADtrigger signal) to the AD conversion unit 22 in accordance with a timingof the Q switch trigger signal, that is, the emission timing of thepulse laser beam. The AD conversion unit 22 starts the sampling of aphotoacoustic signal on the basis of the sampling trigger signal.

FIG. 3 illustrates a configuration example of the birefringent filter56, the driving unit 57, and the rotational displacement detection unit58. In this example, the driving unit 57 is a servo motor, and therotational displacement detection unit 58 is a rotary encoder. Thebirefringent filter 56 rotates in association with the rotation of anoutput axis of the servo motor. The rotary encoder detects therotational displacement of the birefringent filter 56 by a rotatingplate with a slit which is mounted to the output axis of the servo motorand a transmission-type photointerrupter, and generates the BRF stateinformation. For example, the trigger control circuit 30 monitors theBRF state information, and controls a voltage or the like to be suppliedto the servo motor based on the BRF control signal so that the amount ofrotational displacement of the rotation axis of the servo motor, whichis detected by the rotary encoder during a predetermined period of timeis maintained at a predetermined amount, thereby rotating thebirefringent filter 56 at a predetermined speed.

FIG. 4 illustrates an example of a wavelength transmissioncharacteristic (oscillation wavelength characteristic) with respect tothe rotational displacement of the birefringent filter 56. Thebirefringent filter 56 changes an oscillation wavelength of an opticalresonator, for example, between 700 nm and 840 nm. For example, thebirefringent filter 56 repeats a free spectral range (FSR) three timesbetween the rotational displacement positions of 0° and 90° (in ¼rotation), and repeats the FSR twelve times for each rotation.

FIG. 5 illustrates an oscillation wavelength characteristic when theabove-mentioned birefringent filter 56 is rotated at a speed of onerotation per second. When the birefringent filter 56 which has awavelength transmission characteristic illustrated in FIG. 4 is rotatedat a speed of one rotation per one second, the birefringent filter 56repeats the FSR three times for ¼ seconds, and repeats the FSR twelvetimes (12 Hz) per second. As the rotation speed of the birefringentfilter 56 increases, the number of times of repetition of the FSR persecond is increased, and as the rotation speed thereof decreases, thenumber of times of repetition of the FSR per second is decreased.

FIG. 6 is a timing chart illustrating various types of triggers and anemission timing. (a) of FIG. 6 illustrates an oscillation wavelengthcharacteristic (transmission wavelength characteristic of thebirefringent filter 56) of an optical resonator with respect to a timechange. (b) of FIG. 6 illustrates a flash lamp trigger, and (c) of FIG.6 illustrates a Q switch trigger. (d) of FIG. 6 illustrates an emissiontiming of a flash lamp and an emission timing of a pulse laser beam.Meanwhile, in FIG. 5, for the purpose of simplifying the description, adescription is given on the assumption that the flash lamp 52 and the Qswitch 55 instantaneously respond to a trigger, but actually a delaytime is present. However, since the delay is approximately several μseconds to 100μ seconds, the delay is negligible.

First, the trigger control circuit 30 outputs the flash lamp triggersignal to the flash lamp 52 at time t1 in order to cause a pulse laserbeam having a wavelength of 750 nm to be emitted from the laser sourceunit 13 ((b) of FIG. 6), and turns on the flash lamp 52 ((d) of FIG. 6).Thereafter, the trigger control circuit 30 outputs the Q switch triggersignal at time t2 when the rotational displacement position of thebirefringent filter 56 is set to a position corresponding to thewavelength of 750 nm ((c) of FIG. 6), and turns on the Q switch 55 tocause the pulse laser beam having a wavelength of 750 nm to be emittedfrom the optical resonator.

Subsequently, the trigger control circuit 30 outputs the flash lamptrigger signal to the flash lamp 52 at time t3 in order to cause a pulselaser beam having a wavelength of 800 to be emitted from the lasersource unit 13 ((b) of FIG. 6), and turns on the flash lamp 52 ((d) ofFIG. 6). Thereafter, the trigger control circuit 30 outputs the Q switchtrigger signal at time t4 when the rotational displacement position ofthe birefringent filter 56 is set to a position corresponding to thewavelength of 800 nm ((c) of FIG. 6), and turns on the Q switch 55 tocause the pulse laser beam having a wavelength of 800 nm to be emittedfrom the optical resonator.

Here, the time t1 when the flash lamp trigger signal is output is a timeobtained by subtracting a time required for the excitation of the laserrod 51 from the time t2 when the rotational displacement position of thebirefringent filter 56 is set to the position corresponding to thewavelength of 750 nm. In addition, the time t3 when the flash lamptrigger signal is output is a time obtained by subtracting a timerequired for the excitation of the laser rod 51 from the time t4 whenthe rotational displacement position of the birefringent filter 56 isset to the position corresponding to the wavelength of 800 nm. Therotational displacement positions of the birefringent filter 56 whichcorrespond to the time t1 and the time t3 can be obtained from therotational displacement positions of the birefringent filter 56 whichcorrespond to the wavelengths of 750 nm and 800 nm, the rotation speedof the birefringent filter 56, and time required for the excitation ofthe laser rod 51.

Hereinafter, similarly, the trigger control circuit 30 outputs the flashlamp trigger signal to the flash lamp 52 at time t5, time t7, time t9,and time t11. In addition, the trigger control circuit outputs the Qswitch trigger signal to the Q switch 55 at time t6, time t8, time t10,and time t12, and causes the pulse laser beam having a wavelengthdepending on the transmission wavelength of the birefringent filter 56at each time to be emitted. The transmission wavelengths of thebirefringent filter 56 at time t6 and time t10 are 750 nm, and thetransmission wavelengths of the birefringent filter at time t8 and timet12 are 800 nm, and thus the laser source unit 13 sequentially andrepeatedly emits the pulse laser beams having wavelengths of 750 nm and800 nm in this order.

In the example of FIG. 6, the laser source unit 13 alternately emits twopulse laser beams of a pulse laser beam having a wavelength of 750 nmand a pulse laser beam having a wavelength of 800 nm for 1/12 seconds.The laser source unit 13 emits the pulse laser beam twenty-four timesper second while switching the two wavelengths (24 Hz operation). Inother words, the pulse laser beam having a set of two wavelengths isemitted in units of twelve sets per second.

FIG. 7 illustrates an operation procedure of the photoacoustic imagegeneration apparatus 10. Herein, a description will be given on theassumption that a region of a test object which is irradiated with alaser beam is divided into a plurality of partial regions. The triggercontrol circuit 30 outputs the BRF control signal for rotating thebirefringent filter 56 within the laser source unit 13 at apredetermined rotation speed to the laser source unit 13, prior to theirradiation with the pulse laser beam with respect to the test object(step A1). For example, when the birefringent filter 56 repeats an FSRtwelve times during one rotation and the pulse laser beam having awavelength of 750 nm and the pulse laser beam having a wavelength of 800nm are sequentially emitted for 1/12 seconds (in the case of 24 Hzoperation), the trigger control circuit 30 outputs the BRF controlsignal for rotating the birefringent filter 56 once per second.

When the photoacoustic signal is ready to be received, the triggercontrol circuit 30 outputs the flash lamp trigger signal to the lasersource unit 13 at a predetermined timing in order to cause the pulselaser beam having a first wavelength (750 nm) constituting a wavelengthsequence to be emitted (step A2). The flash lamp 52 of the laser sourceunit 13 is turned on in response to the flash lamp trigger signal, andthus the laser rod 51 starts to be excited (step A3). The triggercontrol circuit 30 turns on the flash lamp 52, for example, at a timingcalculated back from a timing at which the rotational displacementposition of the birefringent filter 56 is set to the positioncorresponding to the wavelength of 750 nm, on the basis of the BRF stateinformation.

After the flash lamp 52 is turned on, the trigger control circuit 30turns on the Q switch 55 at a timing when the rotational displacementposition of the birefringent filter 56 is set to the positioncorresponding to the first wavelength (750 nm) constituting thewavelength sequence, on the basis of the BRF state information (stepA4). The laser source unit 13 emits the pulse laser beam having awavelength of 750 nm by the Q switch 55 being turned on.

The pulse laser beam having a wavelength of 750 nm which is emitted fromthe laser source unit 13 is guided to, for example, the probe 11, and afirst partial region of the test object is irradiated with the pulselaser beam from the probe 11. A light absorber absorbs energy of theirradiated pulse laser beam within the test object, and thus aphotoacoustic signal is generated. The probe 11 detects thephotoacoustic signal generated within the test object. The photoacousticsignal detected by the probe 11 is received by the reception circuit 21.

The trigger control circuit 30 outputs the sampling trigger signal tothe AD conversion unit 22 in accordance with a timing at which the Qswitch trigger signal is output. The AD conversion unit 22 samples thephotoacoustic signal received by the reception circuit 21 with apredetermined sampling period (step A5). The photoacoustic signalsampled by the AD conversion unit 22 is stored as first photoacousticdata in the reception memory 23.

The control unit 31 determines whether a remaining wavelength to beemitted is present or not, in other words, whether the pulse laser beamsof all the predetermined wavelengths constituting the wavelengthsequence have been emitted or not (step A6). When a remaining wavelengthis present, the process returns to step A2 in order to emit the pulselaser beam having the next wavelength, and the flash lamp trigger signalis output to the laser source unit 13 from the trigger control circuit30. In step A3, the flash lamp 52 is turned on in response to the flashlamp trigger signal, and in step A4, the trigger control circuit 30turns on the Q switch 55 at a timing when the birefringent filter 56 isset to be at the rotational displacement position corresponding to thesecond wavelength (800 nm) constituting the wavelength sequence, tocause the pulse laser beam to be emitted.

The pulse laser beam having a wavelength of 800 nm which is emitted fromthe laser source unit 13 is guided to, for example, the probe 11, andthe first partial region of the test object is irradiated with the pulselaser beam from the probe 11. The probe 11 detects a photoacousticsignal generated by the light absorber within the test object absorbingthe pulse laser beam having a wavelength of 800 nm. The trigger controlcircuit 30 outputs the sampling trigger signal to the AD conversion unit22 in accordance with the output of the Q switch trigger signal, and theAD conversion unit 22 samples the photoacoustic signal in step A5. Thephotoacoustic signal sampled by the AD conversion unit 22 is stored assecond photoacoustic data in the reception memory 23. The photoacousticimage generation apparatus 10 performs step A1 to step A5 on thewavelengths constituting the wavelength sequence and irradiates the testobject with the pulse laser beam having the wavelengths constituting thewavelength sequence, thereby detecting a photoacoustic signal from thetest object.

When the control unit 31 determines in step A6 that a remainingwavelength is not present, the control unit determines whether all thepartial regions have been selected (step A7). When the partial region tobe selected remains, the process returns to step A2. The photoacousticimage generation apparatus 10 performs step A2 to step A6 on eachpartial region, sequentially irradiates each partial region with pulselaser beams having the wavelengths (750 nm and 800 nm) constituting thewavelength sequence, and stores the first photoacoustic data and thesecond photoacoustic data which correspond to each partial region, inthe reception memory 23. When the irradiation with the pulse laser beamand the detection of the photoacoustic signal are performed on all thepartial regions, photoacoustic data required to generate a photoacousticimage of one frame is gathered.

When the control unit 31 determines in step A7 that all the partialregions have been selected, the process proceeds to the generation ofthe photoacoustic image. The complex number creation unit 24 reads outthe first photoacoustic data and the second photoacoustic data from thereception memory 23, and generates complex number data in which firstphotoacoustic image data is set to a real part and second photoacousticimage data is set to an imaginary part (step A8). The photoacousticimage reconstruction unit 25 performs image reconstruction from thecomplex number data generated in step A8, using a Fourier transformmethod (FTA method) (step A9).

The phase information extraction unit 26 extracts phase information fromthe reconstructed complex number data (reconstructed image) (step A10).For example, when the reconstructed complex number data is expressed byX+iY, the phase information extraction unit 26 extracts the relation ofθ=tan⁻¹(Y/X) as the phase information (but, when the relation of X=0 issatisfied, the relation of θ=90° is satisfied). The intensityinformation extraction unit 27 extracts intensity information from thereconstructed complex number data (step A11). For example, when thereconstructed complex number data is expressed by X+iY, the intensityinformation extraction unit 27 extracts (X²+Y²)^(1/2) as the intensityinformation.

The detection and logarithmic transformation unit 28 performs adetection and logarithmic transformation process on the intensityinformation extracted in step A11. The photoacoustic image constructionunit 29 generates a photoacoustic image on the basis of the phaseinformation extracted in step A10 and the performing of the detectionand logarithmic transformation process, on the intensity informationextracted in step A11 (step A12). For example, the photoacoustic imageconstruction unit 29 generates the photoacoustic image by determiningluminance (gradation value) of each pixel in a distribution image of alight absorber on the basis of the intensity information and bydetermining color of each pixel on the basis of the phase information.The generated photoacoustic image is displayed on the image display unit14.

Here, the rotation speed of the birefringent filter 56 may beappropriately determined depending on the number of wavelengths includedin a wavelength sequence of a pulse laser beam to be emitted.Hereinafter, a case where the wavelength sequence includes sixwavelengths (720 nm, 740 nm, 760 nm, 780 nm, 800 nm, and 820 nm) will bedescribed. FIG. 8 is a timing chart illustrating various types oftriggers and an emission timing when a wavelength sequence includes sixwavelengths. (a) of FIG. 8 illustrates an oscillation wavelengthcharacteristic (transmission wavelength characteristic of thebirefringent filter 56) of an optical resonator with respect to a timechange. (b) of FIG. 8 illustrates a flash lamp trigger, and (c) of FIG.8 illustrates a Q switch trigger. (d) of FIG. 8 illustrates an emissiontiming of a flash lamp and an emission timing of a pulse laser beam.

As illustrated in FIG. 4, when the birefringent filter 56 repeating anFSR twelve times in one rotation is rotated once for four seconds, thebirefringent filter 56 is rotated ¼ per second and repeats the FSR threetimes per second ((a) of FIG. 8). First, the trigger control circuit 30outputs the flash lamp trigger signal to the flash lamp 52 at time t21in order to cause a pulse laser beam having a wavelength of 720 nm to beemitted from the laser source unit 13 ((b) of FIG. 8), and turns on theflash lamp 52 ((d) of FIG. 8). Thereafter, the trigger control circuit30 outputs the Q switch trigger signal at time t22 when the rotationaldisplacement position of the birefringent filter 56 is set to theposition corresponding to the wavelength of 720 nm ((c) of FIG. 8), andcauses the pulse laser beam having a wavelength of 720 nm to be emittedfrom the optical resonator by the Q switch 55 being turned on.

Subsequently, the trigger control circuit 30 outputs the flash lamptrigger signal to the flash lamp 52 at time t23 in order to cause apulse laser beam having a wavelength of 740 nm to be emitted from thelaser source unit 13 ((b) of FIG. 8), and turns on the flash lamp 52((d) of FIG. 8). Thereafter, the trigger control circuit 30 outputs theQ switch trigger signal at time t24 when the rotational displacementposition of the birefringent filter 56 is set to the positioncorresponding to the wavelength of 740 nm ((c) of FIG. 8), and causesthe pulse laser beam having a wavelength of 740 nm to be emitted fromthe optical resonator by the Q switch 55 being turned on.

Hereinafter, similarly, the trigger control circuit 30 outputs the flashlamp trigger signal to the flash lamp 52 at time t25, time t27, timet29, and time t31. In addition, the trigger control circuit outputs theQ switch trigger signal to the Q switch 55 at time t26, time t28, timet30, and time t32, and causes the pulse laser beam having a wavelengthdepending on the transmission wavelength of the birefringent filter 56at each time to be emitted. The transmission wavelengths of thebirefringent filter 56 at time t26, time t28, time t30, and time t32 are760 nm, 780 nm, 800 nm, and 820 nm, respectively, and the laser sourceunit 13 emits six pulse laser beams having a wavelength increasing by 20nm for ⅓ seconds in a range between 720 nm and 820 nm.

In an example of FIG. 8, the laser source unit 13 emits a pulse laserbeam having six wavelengths of 720 nm to 820 nm for ⅓ seconds. The lasersource unit 13 emits the pulse laser beam eighteen times per secondwhile switching the six wavelengths (18 Hz operation). In other words,the pulse laser beam having a set of six wavelengths is emitted in unitsof three sets per second.

It is preferable that the rotation speed of the birefringent filter 56be set so that a pulse laser beam having wavelengths constituting awavelength sequence can be emitted in one FSR. For example, when thenumber of times of the FSR repeated by the birefringent filter 56 duringone rotation is set to k[times/rotation], the number of wavelengthsincluded in the wavelength sequence is set to n[pieces], and the numberof times of emission of the pulse laser beam per unit time is set tom[times/second], the rotation speed of the birefringent filter 56 can beset to a value determined by the relation ofv=m/(k×n)[rotations/second]. In this case, m pulse lasers can be emittedper second while switching n wavelengths for each FSR (m Hz operation).

In this embodiment, the flash lamp 52 is turned on to excite the laserrod 51 while rotating the birefringent filter 56 at a predeterminedrotation speed. After the excitation of the laser rod, the Q switch 55is turned on at a timing when the rotational displacement position ofthe birefringent filter 56 is set to a position corresponding to awavelength of a pulse laser beam to be emitted. As the rotation speed ofthe birefringent filter 56 decreases, for example, the number ofoscillation wavelengths capable of being selected in one FSR of thebirefringent filter 56 can be increased. On the other hand, when thenumber of wavelengths included in the wavelength sequence is two, thespeed of the switching of the two wavelengths can be increased byincreasing the rotation speed of the birefringent filter 56. In thismanner, in this embodiment, it is possible to emit a pulse laser beam ina desired wavelength sequence from the laser source unit 13 bycontrolling the rotation speed of the birefringent filter 56. In thisembodiment, the Q switch trigger signal is output from the ultrasonicwave unit 12, and thus it is not necessary to acquire information suchas a synchronization signal indicating a laser emission timing from thelaser source unit 13.

In this embodiment, complex number data is generated in which one of thefirst photoacoustic data and the second photoacoustic data which areobtained at two wavelengths is set to a real part and the other is setto an imaginary part, and a reconstructed image is generated from thecomplex number data using a Fourier transform method. In this case, itis possible to effectively perform the reconstruction as compared with acase where the first photoacoustic data and the second photoacousticdata are separately reconstructed. A pulse laser beam of a plurality ofwavelengths is irradiated, and a photoacoustic signal (photoacousticdata) at the time of the irradiation with a pulse laser beam having eachwavelength is used, and thus it is possible to perform functionalimaging using optical absorption properties of the respective lightabsorbers being different from each other depending on wavelengths.

In addition, in this embodiment, for example, when a light irradiationregion is divided into three partial regions, a first partial region issequentially irradiated with a pulse laser beam having a firstwavelength and a pulse laser beam having a second wavelength, and asecond partial region is sequentially irradiated with the pulse laserbeam having the first wavelength and the pulse laser beam having thesecond wavelength, and then a third partial region is sequentiallyirradiated with the pulse laser beam having the first wavelength and thepulse laser beam having the second wavelength. In this embodiment, anypartial region is continuously irradiated with the pulse laser beamhaving the first wavelength and the pulse laser beam having the secondwavelength, and then the irradiation moves to the next partial region.In this case, it is possible to shorten the time from the irradiationwith the pulse laser beam having the first wavelength to the irradiationwith the second wavelength at the same position, as compared with a casewhere the three partial regions are irradiated with the pulse laser beamhaving the first wavelength and are then irradiated with the pulse laserbeam having the second wavelength. It is possible to suppressmismatching between the first photoacoustic data and the secondphotoacoustic data by shortening the time between the irradiation withthe pulse laser beam having the first wavelength and the irradiationwith the pulse laser beam having the second wavelength.

Subsequently, a second embodiment of the invention will be described.FIG. 9 illustrates a photoacoustic image generation apparatus accordingto the second embodiment of the invention. In a photoacoustic imagegeneration apparatus 10 a according to this embodiment, an ultrasonicwave unit 12 a includes data separation unit 32, ultrasonic imagereconstruction unit 33, detection and logarithmic transformation unit34, ultrasonic image construction unit 35, image synthesis unit 36, anda transmission control circuit 37, in addition to the configuration ofthe ultrasonic wave unit 12 in the photoacoustic image generationapparatus 10 according to the first embodiment which is illustrated inFIG. 1. The photoacoustic image generation apparatus 10 a according tothis embodiment is different from that in the first embodiment in thatthe apparatus generates an ultrasonic image in addition to aphotoacoustic image. Other parts may be the same as those in the firstembodiment.

In this embodiment, a probe 11 outputs (transmits) ultrasonic waves to atest object and detects (receives) reflected ultrasonic waves from thetest object with respect to the transmitted ultrasonic waves, inaddition to the detection of a photoacoustic signal. A trigger controlcircuit 30 transmits an ultrasonic wave transmission trigger signal forinstructing the transmission of ultrasonic waves to the transmissioncontrol circuit 37 at the time of the generation of an ultrasonic image.When the transmission control circuit 37 receives the trigger signal,the transmission control circuit causes ultrasonic waves to betransmitted from the probe 11. The probe 11 detects reflected ultrasonicwaves from the test object after the transmission of the ultrasonicwaves.

The reflected ultrasonic waves detected by the probe 11 are input to ADconversion unit 22 through a reception circuit 21. The trigger controlcircuit 30 transmits a sampling trigger signal to the AD conversion unit22 in accordance with the transmission timing of the ultrasonic waves,and starts to sample the reflected ultrasonic waves. The AD conversionunit 22 stores sampling data of the reflected ultrasonic waves(reflected ultrasonic data) in the reception memory 23.

The data separation unit 32 separates the reflected ultrasonic datastored in the reception memory 23 and first and second photoacousticdata from each other. The data separation unit 32 transmits thereflected ultrasonic data to the ultrasonic image reconstruction unit33, and transmits the first and second photoacoustic data to complexnumber creation unit 24. The generation of the photoacoustic image onthe basis of the first and second photoacoustic data is the same as thatin the first embodiment. The data separation unit 32 inputs samplingdata of the separated reflected ultrasonic waves to the ultrasonic imagereconstruction unit 33.

The ultrasonic image reconstruction unit 33 generates pieces of data oflines of the ultrasonic image on the basis of reflected ultrasonic waves(sampling data thereof) which are detected by a plurality of ultrasonicvibrators of the probe 11. For example, the ultrasonic imagereconstruction unit 33 adds data from 64 ultrasonic vibrators of theprobe 11 on the basis of a delay time depending on the position of theultrasonic vibrator to generate data for one line (delay additionmethod).

The detection and logarithmic transformation unit 34 obtains an envelopeof the pieces of data of the lines which are output by the ultrasonicimage reconstruction unit 33, and performs logarithmic transformation onthe obtained envelope. The ultrasonic image construction unit 35generates an ultrasonic image on the basis of the data of the lines onwhich the logarithmic transformation is performed. The ultrasonic imagereconstruction unit 33, the detection and logarithmic transformationunit 34, and the ultrasonic image construction unit 35 constituteultrasonic image generation unit that generates an ultrasonic image onthe basis of reflected ultrasonic waves.

The image synthesis unit 36 synthesizes the photoacoustic image and theultrasonic image. For example, the image synthesis unit 36 performsimage synthesis by superimposing the photoacoustic image and theultrasonic image on each other. At this time, it is preferable that theimage synthesis unit 36 perform positioning so that corresponding pointsof the photoacoustic image and the ultrasonic image are set to be at thesame position. The synthesized image is displayed on image display unit14. It is also possible to display the photoacoustic image and theultrasonic image on the image display unit 14 side by side withoutperforming image synthesis, or to switch and display the photoacousticimage and the ultrasonic image.

FIG. 10 illustrates an operation procedure of the photoacoustic imagegeneration apparatus 10 a. Hereinafter, a description will be given onthe assumption that a region of a test object which is irradiated with alaser beam is divided into a plurality of partial regions. The triggercontrol circuit 30 outputs the BRF control signal for rotatingbirefringent filter 56 within a laser source unit 13 at a predeterminedrotation speed to the laser source unit 13 (step B1).

When a photoacoustic signal is ready to be received, the trigger controlcircuit 30 outputs a flash lamp trigger signal in order to cause a pulselaser beam having a first wavelength (750 nm) constituting a wavelengthsequence to be emitted (step B2). A flash lamp 52 is turned on inresponse to the flash lamp trigger signal, and thus a laser rod 51starts to be excited (step B3).

After the flash lamp 52 is turned on, the trigger control circuit 30turns on a Q switch 55 at a timing when a rotational displacementposition of the birefringent filter 56 is set to the positioncorresponding to the first wavelength (750 nm) constituting thewavelength sequence, on the basis of the BRF control signal (step B4).The laser source unit 13 emits a pulse laser beam having a wavelength of750 nm by the Q switch 55 being turned on.

The pulse laser beam having a wavelength of 750 nm which is emitted fromthe laser source unit 13 is guided to, for example, the probe 11, and afirst partial region of the test object is irradiated with the pulselaser beam from the probe 11. A light absorber absorbs energy of theirradiated pulse laser beam within the test object, and thus aphotoacoustic signal is generated. The probe 11 detects thephotoacoustic signal generated within the test object. The triggercontrol circuit 30 outputs a sampling trigger signal to the ADconversion unit 22 in accordance with the output of a Q switch triggersignal. The AD conversion unit 22 receives the photoacoustic signaldetected by the probe 11 through the reception circuit 21, and samplesthe photoacoustic signal with a predetermined sampling period (step B5).The photoacoustic signal sampled by the AD conversion unit 22 is storedas first photoacoustic data in the reception memory 23.

The control unit 31 determines whether a remaining wavelength ispresent, in other words, whether the pulse laser beams of all thewavelengths constituting the wavelength sequence have been emitted ornot (step B6). When a remaining wavelength is present, the processreturns to step B2 in order to emit the pulse laser beam having the nextwavelength, and the flash lamp trigger signal is output to the lasersource unit 13 from the trigger control circuit 30. In step B3, theflash lamp 52 is turned on in response to the flash lamp trigger signal,and in step B4, the trigger control circuit 30 turns on the Q switch 55at a timing when the birefringent filter 56 is set to be at therotational displacement position corresponding to the second wavelength(800 nm) constituting the wavelength sequence, to cause the pulse laserbeam to be emitted.

The pulse laser beam having a wavelength of 800 nm which is emitted fromthe laser source unit 13 is guided to, for example, the probe 11, andthe first partial region of the test object is irradiated with the pulselaser beam from the probe 11. The probe 11 detects the photoacousticsignal generated by the light absorber within the test object absorbingthe pulse laser beam having a wavelength of 800 nm. The trigger controlcircuit 30 outputs the sampling trigger signal to the AD conversion unit22 in accordance with the output of the Q switch trigger signal, and theAD conversion unit 22 samples the photoacoustic signal in step B5. Thephotoacoustic signal sampled by the AD conversion unit 22 is stored assecond photoacoustic data in the reception memory 23. The photoacousticimage generation apparatus 10 performs step B1 to step B5 to eachwavelengths constituting the wavelength sequence, and irradiates thetest object with the pulse laser beam having each wavelengthsconstituting the wavelength sequence, thereby detecting a photoacousticsignal from the test object. The step B1 to step B5 may be the same asstep A1 to step AS of FIG. 7.

When the control unit 31 determines in step B6 that a remainingwavelength is not present, the process proceeds to the transmission andreception of ultrasonic waves. The trigger control circuit 30 transmitsthe ultrasonic waves to the test object from the probe 11 through thetransmission control circuit 37 (step B7). In step B7, the ultrasonicwaves are transmitted to the same region as the partial region of thetest object which is irradiated with the pulse laser beam. The probe 11detects reflected ultrasonic waves with respect to the transmittedultrasonic waves (step B8). The detected reflected ultrasonic waves aresampled in the AD conversion unit 22 through the reception circuit 21,and are stored as reflected ultrasonic data in the reception memory 23.

The control unit 31 determines whether all the partial regions have beenselected (step B9). When the partial region to be selected remains, theprocess returns to step B2. The photoacoustic image generation apparatus10 a performs step B2 to step B6 on each partial region, sequentiallyirradiates each partial region with pulse laser beams having thewavelengths (750 nm and 800 nm) constituting the wavelength sequence,and stores the first photoacoustic data and the second photoacousticdata in the reception memory 23. In addition, step B7 and step B8 areperformed to store the reflected ultrasonic data in the reception memory23. When the irradiation with the pulse laser beam, the detection of thephotoacoustic signal, and the transmission and reception of theultrasonic waves are performed on all the partial regions, data requiredto generate a photoacoustic image and an ultrasonic image of one frameis gathered.

When the control unit 31 determines in step B9 that all the partialregions have been selected, the process proceeds to the generation ofthe photoacoustic image and the ultrasonic image. The data separationunit 32 separates the first and second photoacoustic data and thereflected ultrasonic data from each other. The data separation unit 32transmits the separated first and second photoacoustic data to thecomplex number creation unit 24, and transmits the reflected ultrasonicdata to the ultrasonic image reconstruction unit 33. The complex numbercreation unit 24 generates complex number data in which firstphotoacoustic image data is set to a real part and second photoacousticimage data is set to an imaginary part (step B10). The photoacousticimage reconstruction unit 25 performs image reconstruction from thecomplex number data generated in step B10, using a Fourier transformmethod (FTA method) (step B11).

The phase information extraction unit 26 extracts phase information fromthe reconstructed complex number data (step B12). The intensityinformation extraction unit 27 extracts intensity information from thereconstructed complex number data (step B13). The detection andlogarithmic transformation unit 28 performs a detection and logarithmictransformation process on the intensity information extracted in stepB13. The photoacoustic image construction unit 29 generates aphotoacoustic image on the basis of the phase information extracted instep B12 and the performing of the detection and logarithmictransformation process, on the intensity information extracted in stepB13 (step B14). Here, step B10 to step B14 may be the same as step A8 tostep A12 of FIG. 7.

The ultrasonic image reconstruction unit 33 generates pieces of data oflines of the ultrasonic image using, for example, a delay additionmethod. The detection and logarithmic transformation unit 34 obtains anenvelope of the pieces of data of the lines which are output by theultrasonic image reconstruction unit 33, and performs logarithmictransformation on the obtained envelope. The ultrasonic imageconstruction unit 35 generates an ultrasonic image on the basis of thepieces of data of the lines on which the logarithmic transformation isperformed (step B15). The image synthesis unit 36 synthesizes thephotoacoustic image and the ultrasonic image and displays thesynthesized image on the image display unit 14 (step B16).

In this embodiment, the photoacoustic image generation apparatusgenerates an ultrasonic image in addition to a photoacoustic image. Itis possible to observe a portion not capable of being formed as an imagein the photoacoustic image by referring to the ultrasonic image. Othereffects are the same as those in the first embodiment.

Meanwhile, in the above-described embodiments, an example in which firstphotoacoustic data and second photoacoustic data are created as complexnumbers has been described, but the first photoacoustic data and thesecond photoacoustic data may be separately reconstructed without beingcreated as complex numbers. Furthermore, herein, a ratio between thefirst photoacoustic data and the second photoacoustic data is calculatedby the creation of complex numbers and by using phase information, butthe same effect is obtained even though the ratio is calculated fromintensity information of both the pieces of data. In addition, theintensity information can be generated on the basis of signal intensityin a first reconstructed image and signal intensity in a secondreconstructed image.

In the generation of a photoacoustic image, the number of wavelengths ofa pulse laser beam with which a test object is to be irradiated is notlimited two, and the test object may be irradiated with three or morepulse laser beams, and thus the photoacoustic image may be generated onthe basis of pieces of photoacoustic data corresponding to therespective wavelengths. In this case, for example, the phase informationextraction unit 26 may generate a magnitude relation between relativesignal intensities of the pieces of photoacoustic data corresponding tothe respective wavelengths, as phase information. In addition, theintensity information extraction unit 27 may generate the signalintensities in the pieces of photoacoustic data corresponding to therespective wavelengths, which are grouped into one, as intensityinformation.

In the above-described embodiments, a description has been made on theassumption that the trigger control circuit 30 monitors BRF stateinformation and controls a rotation speed of the birefringent filter 56to have a predetermined rotation speed on the basis of the BRF controlsignal, but is not limited thereto. FIG. 11 illustrates a modifiedexample of a laser source unit. A laser source unit 13 a includes arotation control unit 59 in addition to the configuration of the lasersource unit 13 illustrated in FIG. 2. The rotation control unit 59controls a voltage or the like to be supplied to driving unit 57 so thatthe amount of rotational displacement which is detected by rotationaldisplacement detection unit 58 during a predetermined period of time isset to an amount according to a predetermined rotation speed of thebirefringent filter 56. The trigger control circuit 30 instructs therotation control unit 59 on the rotation speed of the birefringentfilter 56 on the basis of the BRF control signal. The rotation controlunit 59 drives the driving unit 57 so that the rotation speed of thebirefringent filter 56 is set to the instructed rotation speed.

As described above, although the invention has been described on thebasis of the preferred embodiments, the ultrasonic wave unit and thephotoacoustic image generation apparatus of the invention are notlimited to those in the above-described embodiments, and variouscorrections and modifications made to the configurations of theabove-described embodiments may also be included in the scope of theinvention.

What is claimed is:
 1. A photoacoustic image generation apparatuscomprising: a laser source unit that sequentially emits a plurality ofpulse laser beams in a predetermined wavelength sequence having at leasttwo different wavelengths, the laser source unit including a laser rod,an excitation light source that irradiates the laser rod with excitationlight, an optical resonator that has a pair of mirrors facing each otherwith the laser rod interposed therebetween, a Q switch which is insertedinto the optical resonator, and a birefringent filter which is insertedinto the optical resonator and changes an oscillation wavelength of theoptical resonator in association with rotational displacement of thebirefringent filter; and an acoustic wave unit that generates aphotoacoustic image, the acoustic wave unit including a detection unitthat detects a photoacoustic signal generated within an object when theobject is irradiated with the pulse laser beam having each wavelengthincluded in the predetermined wavelength sequence and generates piecesof photoacoustic data corresponding to the respective wavelengths, anintensity ratio extraction unit that extracts a magnitude relationbetween relative signal intensities of the pieces of photoacoustic datacorresponding to the respective wavelengths, a photoacoustic imageconstruction unit that generates the photoacoustic image on the basis ofthe extracted magnitude relation, and a trigger control circuit thatcauses the laser rod to be irradiated with excitation light from theexcitation light source while rotating the birefringent filter at apredetermined rotation speed depending on the number of wavelengthsincluded in the wavelength sequence, and after the irradiation with theexcitation light, turns on the Q switch at a timing when a rotationaldisplacement position of the birefringent filter is set to a positioncorresponding to the wavelength of the pulse laser beam to be emitted tocause the pulse laser beam to be emitted.
 2. The photoacoustic imagegeneration apparatus according to claim 1, wherein the predeterminedrotation speed is determined on the basis of a change characteristic ofthe oscillation wavelength with respect to the rotational displacementposition in the birefringent filter, the number of wavelengths includedin the wavelength sequence, and the number of times of emission of thepulse laser beam per unit time.
 3. The photoacoustic image generationapparatus according to claim 2, wherein when the number of times of afree spectral range repeated during one rotation is set tok[times/rotation], the number of wavelengths included in the wavelengthsequence is set to n[pieces], and the number of times of emission of thepulse laser beam per unit time is set to m[times/second], thepredetermined rotation speed of the birefringent filter is determined asa value calculated by a relation of v=m/(k×n)[rotations/second].
 4. Thephotoacoustic image generation apparatus according to claim 1, whereinthe trigger control circuit continuously rotates the birefringent filterin a predetermined direction at the predetermined rotation speed.
 5. Thephotoacoustic image generation apparatus according to claim 1, whereinthe trigger control circuit determines a timing at which the excitationlight is irradiated and a timing at which the Q switch is turned on, onthe basis of birefringent filter state information indicating therotational displacement position of the birefringent filter.
 6. Thephotoacoustic image generation apparatus according to claim 5, whereinwhen the birefringent filter state information is set to informationindicating a position obtained by subtracting the amount of rotationaldisplacement of the birefringent filter during a period of time requiredfor the excitation of the laser rod from a position of the birefringentfilter which corresponds to the wavelength of the pulse laser beam to beemitted, the trigger control circuit causes the laser rod to beirradiated with excitation light.
 7. The photoacoustic image generationapparatus according to claim 5, wherein the trigger control circuitrotates the birefringent filter so that the amount of change in thebirefringent filter state information during a predetermined period oftime is set to the amount of change depending on the predeterminedrotation speed.
 8. The photoacoustic image generation apparatusaccording to claim 1, wherein the laser source unit further includes adriving unit that rotates the birefringent filter, a rotationaldisplacement detection unit that detects the rotational displacement ofthe birefringent filter, and a rotation control unit that controls thedriving unit so that the amount of rotational displacement of thebirefringent filter which is detected by the rotational displacementdetection unit during a predetermined period of time is set to an amountdepending on the predetermined rotation speed.
 9. The photoacousticimage generation apparatus according to claim 8, wherein the rotationaldisplacement detection unit outputs a birefringent filter state signalindicating the rotational displacement position of the birefringentfilter to the acoustic wave unit.
 10. The photoacoustic image generationapparatus according to claim 9, wherein the trigger control circuitoutputs a birefringent filter control signal for controlling rotation ofthe birefringent filter to the driving unit.
 11. The photoacoustic imagegeneration apparatus according to claim 10, wherein the trigger controlcircuit outputs a flash lamp trigger signal for controlling emission ofthe excitation light to the excitation light source on the basis of thebirefringent filter state signal, and causes the laser rod to beirradiated with excitation light from the excitation light source. 12.The photoacoustic image generation apparatus according to claim 9,wherein the trigger control circuit outputs a Q switch trigger signalfor turning on the Q switch when the birefringent filter state signal isset to signal indicating a position of the birefringent filter whichtransmits a wavelength of a pulse laser beam to be emitted, and causesthe pulse laser beam to be emitted from the laser source unit.
 13. Thephotoacoustic image generation apparatus according to claim 12, whereinthe acoustic wave unit includes an AD converting unit that performssampling of the photoacoustic signal with a predetermined samplingperiod, and wherein the trigger control circuit outputs a samplingtrigger signal indicating a sampling timing of the AD converting unit tothe AD converting unit, in synchronization with an output timing of theQ switch trigger signal.
 14. The photoacoustic image generationapparatus according to claim 1, wherein the acoustic wave unit furtherincludes an intensity information extraction unit that generatesintensity information indicating signal intensity on the basis of thepieces of photoacoustic data corresponding to the respectivewavelengths, and wherein the photoacoustic image construction unitdetermines a gradation value of each pixel of the photoacoustic image onthe basis of the intensity information and determines a display color ofeach pixel on the basis of the extracted magnitude relation.
 15. Thephotoacoustic image generation apparatus according to claim 14, whereinthe predetermined wavelength sequence includes a first wavelength and asecond wavelength, wherein the acoustic wave unit further includes acomplex number creation unit that generates complex number data in whichone of first photoacoustic data corresponding to a photoacoustic signal,detected when irradiation with the pulse laser beam having the firstwavelength is performed, and second photoacoustic data corresponding toa photoacoustic signal, detected when irradiation with the pulse laserbeam having the second wavelength is performed, is set to a real partand the other one is set to an imaginary part, and a photoacoustic imagereconstruction unit that generates a reconstructed image from thecomplex number data using a Fourier transform method, and wherein theintensity ratio extraction unit extracts phase information as themagnitude relation from the reconstructed image, and the intensityinformation extraction unit extracts the intensity information from thereconstructed image.
 16. The photoacoustic image generation apparatusaccording to claim 1, wherein the detection unit further detectsreflected acoustic waves with respect to acoustic waves transmitted tothe object to generate reflected acoustic wave data, and wherein theacoustic wave unit further includes an acoustic wave image generationunit that generates an acoustic wave image on the basis of the reflectedacoustic wave data.
 17. An acoustic wave unit comprising: a detectionunit that detects a photoacoustic signal generated within an object whenthe object is irradiated with a pulse laser beam having each wavelengthincluded in a predetermined wavelength sequence including at least twodifferent wavelengths, and generates pieces of photoacoustic datacorresponding to the respective wavelengths; an intensity ratioextraction unit that extracts a magnitude relation between relativesignal intensities of the pieces of photoacoustic data corresponding tothe respective wavelengths; a photoacoustic image construction unit thatgenerates a photoacoustic image on the basis of the extracted magnituderelation; and a trigger control circuit that causes the laser rod to beirradiated with excitation light from an excitation light source whilerotating a birefringent filter which is inserted into an opticalresonator that includes a pair of mirrors facing each other with a laserrod interposed therebetween and changes an oscillation wavelength of theoptical resonator in association with rotational displacement of thebirefringent filter, at a predetermined rotation speed depending on thenumber of wavelengths included in the wavelength sequence, and after theirradiation with the excitation light, turns on a Q switch inserted intothe optical resonator at a timing when a rotational displacementposition of the birefringent filter is set to a position correspondingto the wavelength of the pulse laser beam to be emitted, to cause thepulse laser beam to be emitted.
 18. The acoustic wave unit according toclaim 17, wherein the trigger control circuit receives a birefringentfilter state signal indicating the rotational displacement position ofthe birefringent filter to acquire the rotational displacement positionof the birefringent filter.
 19. The acoustic wave unit according toclaim 17, wherein the trigger control circuit outputs a birefringentfilter control signal for controlling rotation of the birefringentfilter to a driving unit that rotates the birefringent filter.
 20. Theacoustic wave unit according to claim 18, wherein the trigger controlcircuit outputs a flash lamp trigger signal for controlling emission ofthe excitation light source to the excitation light source on the basisof the birefringent filter state signal.
 21. The acoustic wave unitaccording to claim 18, wherein the trigger control circuit outputs a Qswitch trigger signal for turning on the Q switch when the birefringentfilter state signal is set to signal indicating a position of thebirefringent filter which transmits a wavelength of a pulse laser beamto be emitted, and causes the pulse laser beam to be emitted.
 22. Theacoustic wave unit according to claim 21, further comprising an ADconverting unit that performs sampling of the photoacoustic signal witha predetermined sampling period, wherein the trigger control circuitoutputs a sampling trigger signal indicating a sampling timing of the ADconverting unit to the AD converting unit, in synchronization with anoutput timing of the Q switch trigger signal.